One of the various types of optical memory technologies currently in existence is one which is both recordable and erasable and operates on the principle of two thermo-optically active layers which absorb light at different wavelengths and respond to the resulting temperature increase in a reversible manner. The response of one such layer is expansion, while the response of the other is the transition from a glassy state to a rubbery state. The coupling of these two layers together results in the second layer resisting forces exerted by the first when the second layer is in its glassy state, and accommodating such forces when in its rubbery state.
One medium in which this principle is applied is described in European Pat. Application Publication No. 136070, published on Apr. 3, 1985, entitled "Erasable Optical Data Storage Medium and Method and Apparatus for Recording Data on the Medium" (Optical Data, Inc.). In this medium, the two thermo-optically active layers, which are referred to herein as the "expansion" and "retention" layers, respectively, are both supported on a rigid substrate.
Data is recorded by heating the expansion layer by absorption of light from a laser beam at a "record" wavelength to cause the expansion layer to expand away from the substrate and form a protrusion or "bump" extending into the retention layer. While this is occurring, the retention layer receives heat from the expansion layer by conduction and rises in temperature above its glass transition temperature so that it can deform to accommodate the bump. The beam is then turned off and the retention layer cools quickly to its glassy state before the bump levels out, thereby fixing the bump. Reading or playback of the data is achieved by a low intensity "read" beam which is focused on the partially reflecting interface between the retention layer and air. When the read beam encounters the bump, some of the reflected light is scattered, while other portions of the reflected light destructively interfere with reflected light from non-bump areas. The resulting drop in intensity is registered by the detector. Removal of the bump to erase the data is achieved by a second laser beam at an "erase" wavelength which is absorbed by the retention layer. This beam heats the retention layer aloe to a rubbery state where its viscoelastic forces and those of the expansion layer return it to its original flat configuration.
A variation on this system is disclosed in U.S. patent application Ser. No. 153,288 filed on even date herewith, entitled "Optical Data Storage Media For Substrate Incident Recording." inventors B. Clark, J. Finegan, and R. Guerra, commonly assigned herewith. In this variation, a reflective layer which deforms to conform to the data marks is included in addition to the expansion and retention layers. The reflective layer reflects light back through the expansion layer to provide a double pass during data recordation for greater absorption and mark focusing. During playback, focusing is done on the reflective layer rather than an air-retention layer interface, and a higher degree of reflection is achieved.
In both of these systems, the expansion and retention layers are independently absorptive due to dyes which are selective for the record and erase beams. respectively. Accordingly, the retention layer absorbs light only when the erase beam is on. During recordation, the heating of the retention layer to convert it to the rubbery state is achieved by conduction of heat from the expansion layer. The time required for such conductive heat transfer is a limitation on both the speed and the sensitivity of systems of this type.
The present invention provides for light absorption in the retention layer during recordation, in addition to its light absorption during erasure. The result is direct heating of both the expansion and retention layers during recordation, lessening the need for heat transfer between these layers by conduction, and thereby quickening the response of the medium to the record beam. This innovation is applicable to media both with and without a reflective layer.